1. Field of the Invention
The present invention relates to a line voltage regulator, and more specifically to an AC variable voltage source using pulse width modulation techniques to drive boost and buck transformers such as to develop a controlled voltage output despite changes in the input voltage, the load or other operating conditions.
2. General discussion of the background.
The voltage from an unregulated source will invariably deviate from the ideal instantaneous and average values. Deviation may be present on the incoming voltage or it may be induced by loads. Deviation may take the form of (1) long-term variation of instantaneous and average voltage, (2) short-duration transients, and (3) harmonic distortion.
Many methods have been utilized in an attempt to provide controlled output voltage. Regulators utilizing boost and buck transformers are known in the prior art; such regulators have controlled the average value, rather than the instantaneous value, of AC output.
One type of average-controlling regulator is implemented by automatically switching boost and buck transformer taps so as to control their turns ratios. The switched-tap regulator provides output voltage control in discrete steps, the size of which is determined by the number of taps.
The present invention utilizes buck and boost transformers in such a manner that the output voltage is continuously controlled over the adjustment range.
It utilizes separate boost and buck transformers that are operated to alternately provide boost and buck. The frequency of alternation between boost and buck is much higher than the power frequency, which may be in the range of 50 to 400 hertz. In addition, the instantaneous value of output is controlled, rather than the average value.
The present invention utilizes bipolar transistors across the DC terminals of a bridge rectifier to switch to the boost or buck condition.
General state of the art teaches this basic AC switching scheme, which provides for the control of the boost and buck transformers in a different manner, so that the present state of the art appears to be applicable only to a single-phase circuits. For example, Harrison in his U.S. Pat. No. 3,596,172, does in fact regulate the AC voltage by utilizing switches, comprising bipolar transistor and rectifier to drive a single boost/buck transformer. An electronic circuit, as taught by Harrison, controls the average value of AC output voltage. It cannot control the instantaneous value so as to attenuate transients and harmonics. Harrison's device cannot synchronize switching frequency to the power frequency. Further, Harrison's device is only capable of controlling single-phase AC voltage.
Another disadvantage that can be attributed to Harrison's device is that the bipolar transistor base drive circuit is inadequate for transistors which switch at high frequency, since his circuit does not provide reverse-bias of the transistor collector-to-emittor-junction which is necessary for fast turn off.
Another example of the present state of the art is a patent issued to McCartney, U.S. Pat. No. 4,352,055, which utilizes field effect transistors (FET) as AC switches to control a single boost/buck transformer. McCartney teaches the use of a power transformer which provides boost and buck voltage sources and input-to-output isolation. The boost and buck sources are alternatively connected by the FET switches to the primary of the boost/buck transformer.
McCartney's device also suffers some major disadvantages. For example, it requires a power transformer to provide boost and buck voltage sources and to provide isolation between input and output. Further, field effect transistors utilized by McCartney have limited power handling capability. As was the case with Harrison, McCartney's electronic circuit controls the average value of AC output and cannot control the instantaneous value so as to actively attenuate transients and harmonics. McCartney's device cannot sychronize the switching frequency to the power frequency and it is capable of controlling only single-phase AC voltage.